The development of new tools and techniques to investigate cell behavior is vital to the understanding of basic biology and the development of new pharmaceuticals and clinical treatment techniques. Techniques such as flow cytometry and image cytometry have allowed researchers and clinicians to assay populations of cells on the order of 105 to 108 and to categorize these into subpopulations and identify rare events on the order of 1 in 106.
Cytometry is an analytical method capable of precisely quantifying the functional states of individual cells by measuring their optical characteristics based on fluorescence or scattered light. As a quantitative analytical method applicable to individual cells, it has contributed to the progress of cell biology and is now widely used in the basic and clinical study. Cytometry can be classified into two categories, flow cytometry and image cytometry, based on the measurement method. Flow cytometry monitors the properties of cells carried through the detection area in a fluid stream. It has several unique advantages. The most important one is the rapidity of this measurement scheme. With throughput rate up to 100,000 cells per second, the analysis of a large cell population for the detection of a few rare cells, are possible. Also, based on multi-parametric analysis, it is well suited to identify and distinguish the properties of cell sub-populations. Further, cell sorting methods implemented with flow cytometry enables physical selection of a specific cellular sub-population for further analysis and clonal propagation. Therefore, flow cytometry/cell sorting methods are now an indispensable tool in immunology, molecular and cell biology, cytogenetics, and the human genome project.
Image cytometry has been recently introduced as a complementary method for flow cytometry. This method images individual cells plated in a 2-D culture plate. Cellular morphology and biochemical states are typically quantified by fluorescence microscopy. Although the throughput rate of this method is relatively low (approximately 200 cells per second), it has several unique advantages. Individual cells of interest can be re-located so that they can be further analyzed. One key example is the ability of this method to monitor the temporal evolution of a cellular sub-population. Image cytometry also provides cellular structural information, such as the relative distribution of a fluorochrome in the nucleus and in the cytoplasm with micron level resolution.
Two-photon fluorescence microscopy (TPM) is important for biological imaging. This technology enables noninvasive study of biological specimens in three dimensions with submicrometer resolution. Two-photon excitation of fluorophores results from the simultaneous absorption of two photons. This excitation process has a number of unique advantages, such as reduced specimen photodamage and enhanced penetration depth. It also produces higher-contrast images and is a novel method to trigger localized photochemical reactions.
Two-photon microscopy continues to find an increasing number of applications in biology and medicine. In neurobiology studies, TPM has been applied to study the neuron structure and function in intact brain slices, the role of calcium signaling in dendritic spine function, neuronal plasticity and the associating cellular morphological changes, and hemodynamics in rat neocortex. In embryology studies, two-photon imaging has been used to examine, for example, calcium passage during sperm-egg fusion, the origin of bilateral axis in sea urchin embryos, cell fusion events in C. elegans hypodermis, and hamster embryo development.
Further experiments in TPM tissue imaging include the imaging of the relative transparent ex vivo rabbit cornea based on reduced pyridine nucleotides, NAD(P)H. The methods to image more opaque tissues such as skin and intestine are also being refined.
Two-photon microscopy is an important tool for noninvasive biomedical diagnosis. Two-photon excitation is a fluorescence technique providing an opportunity to assess tissue biochemistry and structures down to the depth of several hundred micrometers. Although the clinical potentials of two-photon microscopy have been demonstrated, significant engineering challenges remain in terms of adapting this technology to the clinical setting.
There still remains a need for imaging of thick samples whose axial dimensions extend beyond the practical working depth of a microscope objective. A further obstacle in implementing this technology is speed. Using a traditional confocal microscope, imaging a one cm3 specimen may require upwards of weeks. The slow data acquisition speed makes imaging large specimens in extended depth impractical.